the dynamo. At
the start the residual magnetism of the magnet cores gives a weak field.
The armature coils cut this and pass a current through the circuit. The
magnets are further excited, and the field becomes stronger; and so on
till the dynamo is developing full power. Series winding is used where
the current in the external circuit is required to be very constant.

[Illustration: FIG. 76.--The brushes of a Holmes dynamo.]

Fig. 78 shows another method of winding--the _shunt_. Most of the
current generated passes through the external circuit 2, 2; but a part
is switched through a separate winding for the magnets, denoted by the
fine wire 1, 1. Here the strength of the magnetism does not vary
directly with the current, as only a small part of the current serves
the magnets. The shunt winding is therefore used where the voltage (or
pressure) must be constant.

[Illustration: FIG. 77.--Sketch showing a "series" winding.]

[Illustration: FIG. 78.--"Shunt" winding.]

A third method is a combination of the two already named. A winding of
fine wire passes from brush to brush round the magnets; and there is
also a series winding as in Fig. 77. This compound method is adapted
more especially for electric traction.

ALTERNATING DYNAMOS.

These have their field magnets excited by a separate continuous current
dynamo of small size. The field magnets usually revolve inside a fixed
armature (the reverse of the arrangement in a direct-current generator);
or there may be a fixed central armature and field magnets revolving
outside it. This latter arrangement is found in the great power stations
at Niagara Falls, where the enormous field-rings are mounted on the top
ends of vertical shafts, driven by water-turbines at the bottom of pits
178 feet deep, down which water is led to the turbines through great
pipes, or penstocks. The weight of each shaft and the field-ring
attached totals about thirty-five tons. This mass revolves 250 times a
minute, and 5,000 horse power is constantly developed by the dynamo.
Similar dynamos of 10,000 horse power each have been installed on the
Canadian side of the Falls.

[Illustration: FIG. 79.]

TRANSMISSION OF POWER.

Alternating current is used where power has to be transmitted for long
distances, because such a current can be intensified, or stepped up, by
a transformer somewhat similar in principle to a Ruhmkorff coil _minus_
a contact-breaker (see p. 122). A typical example of tr